1. Field of the Invention
The invention relates to magnetic inductive flow meters.
2. Description of the Background Art
Magnetic inductive flow meters use a measuring method that is based on Faraday's law of electromagnetic induction. The first basis for the magnetic inductive measurement of the flow velocity of fluids was reported in 1832 in a publication by Michael Faraday. Modern electronic switching technology in conjunction with alternating magnetic fields made it possible to overcome the separation of the useful signals, proportional to the flow velocity, from interference signals, which occur in electrochemical processes during the generation of the magnetic field at the electrodes used for signal decoupling. Thus, nothing seemed to stand in the way of the wide industrial use of magnetic inductive flow meters.
The measuring principle of magnetic inductive flow meters utilizes the separation of moving charges in a magnetic field. The conductive fluid to be measured flows through a tube which is made of nonmagnetic material and whose interior is electrically insulated. A magnetic field is applied from the outside by means of coils. The charge carriers present in the conductive fluid, such as ions and other charged particles, are deflected by the magnetic field: the positive charge carriers to one side and the negative charge carriers to another side. A voltage, which is detected with a measuring device, arises owing to the charge separation at measuring electrodes arranged perpendicular to the magnetic field. The value of the measured voltage is proportional to the flow velocity of the charge carriers and thereby proportional to the flow velocity of the measuring fluid. The flow volume can be determined over time by integration.
In magnetic fields generated by pure alternating voltage, induction of interference voltages occurs in the electrodes, which must be suppressed by suitable and costly filters. For this reason, the magnetic field is usually generated by a clocked direct current of alternating polarity. This assures a stable zero point and makes the measurement insensitive to effects by multiphase substances and inhomogeneities in the fluid. In this way, a usable measuring signal can also be achieved at a low conductivity.
If a measuring fluid moves through the measuring tube, according to the induction law a voltage is present at both measuring electrodes, which are arranged in the measuring tube perpendicular to the flow direction and perpendicular to the magnetic field. This voltage in the case of a symmetric flow profile and a homogeneous magnetic field is directly proportional to the average flow velocity. The inductive flow measuring method is capable of generating an electrically usable signal for further processing directly from the flow. The following equation basically applies:U=k*B*D*v 
where U=voltage, k=proportionality factor, B=magnetic field strength, D=tube diameter, and v=flow velocity.
The selection of the proper electrode material is critical for the reliable function and measuring accuracy of the magnetic inductive flow meter. The measuring electrodes are in direct contact with the medium and must therefore be sufficiently corrosion-resistant and ensure good electrical transfer to the measuring fluid. The following are used as electrode materials: stainless steel, CrNi alloys, platinum, tantalum, titanium, and zirconium. Sintered electrodes are also used in the case of measuring sensors with ceramic measuring tubes.
EP 1616152 B1, which corresponds to US Publication No. US 20070022823, which is incorporated herein by reference, and which discloses improved electrodes. These electrodes have a metal, and a salt of the metal is arranged so that it is located between the metal and the fluid, whereby the salt layer is either applied electrochemically or sintered on. Silver as the salt silver chloride or silver fluoride is preferred as the metal. A porous protective element, for example, a glass frit, can be mounted in front of the silver electrode as protection against dirt.
A possible realization of a magnetic inductive flow meter is disclosed in U.S. Pat. No. 6,626,048 B1, which is incorporated by reference. This publication presents the physical and electronic fundamentals.
It is understood that major problems must be solved in the practical realization of a magnetic inductive flow meter.
In one respect, this is a matter of the material. The measuring tube must be amagnetic in order not to interfere with the magnetic field. The measuring tube further must be electrically insulating in order not to interfere with the picking up of the voltage with use of the electrodes. Moreover, the tube should have a food-safe material, when the liquid is a food, for example, drinking water.
These requirements can be fulfilled best when a food-safe plastic is used as the material. Nevertheless, plastics have the disadvantage of a much lower strength compared with metal. Resistance to internal pressure, however, is an essential requirement. The attempt to achieve internal pressure resistance with an increased thickness of the tube wall is not practicable, because otherwise the magnetic field would be weakened too greatly.
Another problem with plastics is water diffusion. This causes swelling of the plastic, as a result of which the dimensions particularly of the measuring channel change, which leads to a deterioration in the measuring accuracy. Water diffusion also greatly reduces the strength of the plastic. In fiber-reinforced plastics, the adhesion between the plastic and fiber is also partially lost.
During measurement of warm and hot fluids, the plastic softens and the strength also declines.
Chemicals, e.g., chlorine, in the measuring fluid can also attack the plastic. This also applies to UV radiation.
Furthermore, the meter housing must be tension-resistant, because considerable tensile stress can occur when a meter is screwed into existing tubing, e.g., in the screw thread. Tensile stress, particularly long-term tensile stress, is damaging to plastics, however, in particular the thinner the plastic material.
During installation on-site, other forces can act on the plastic, which lead to damage, when no provisions have been made by the design engineers and manufacturers.